The government must approve any foreign takeovers worth over $330 million. But CNOOC's close ties to the Chinese government complicated the deal, she said:

...earlier this year, the Canadian Security Intelligence Service warned that firms “with close ties to their home governments have pursued opaque agendas or received clandestine intelligence support for their pursuits here.” So the Nexen takeover might also have triggered review under national security parameters, even though CSIS never specifically mentioned CNOOC.